1. Field of the Invention
The present invention relates to a holographic screen for displaying images by diffracting and dispersing an image beam projected by an image projector.
2. Description of the Related Art
Holographic screens for displaying images by diffracting and dispersing an image beam projected by an image projector are known in the prior art.
The hologram elements used in such a holographic screen are produced by recording a diffuser 98 on a photosensitive material 95 by means of an exposure optical system 9 such as that shown in FIG. 20.
In this system a laser beam 900 (of a wavelength of e.g. 514.5 nm) emitted from a laser (e.g. an Ar laser) oscillator 90 is separated into two directions by a semi-transparent mirror 990.
One of the separated beams is passed through two reflective mirrors 991 and is split into divergent beams by an objective lens 931, and then, is projected onto the photosensitive material 95. These divergent beams are reference beams 96.
The other of the separated beams is passed through another two reflective mirrors 992 and is split by an objective lens 932, and then, is guided to a parabolic mirror 99. The beams reflected by this parabolic mirror 99, after passing through the diffuser 98 to make diffused beams, are projected onto the photosensitive material 95. The diffused beams from the diffuser 98 are object beams 97.
The object beams 97 and reference beams 96 interfere with each other when the photosensitive material 95 is exposed to them. As a result, the diffuser 98 is recorded on the photosensitive material 95 to produce a hologram element.
In a holographic screen utilizing hologram elements produced in this way, the image projected from the image projector takes on a greenish hue, and the like, giving rise to the problem that the color of the projected image is not satisfactorily reproduced.
In response to this problem, as a method for improving the color reproducibility of the holographic screen, a method of laminating a directional dispersion film onto the holographic elements is disclosed in Unexamined Patent Publication (Kokai) No. 11-202417. Namely, a visibility control film “Lumisty” produced by Sumitomo Chem. Co. Ltd, serving as the above directional dispersion film, is laminated onto the holographic elements, for example.
Consequently, a given width is imparted on the angle of incidence of the image beams transmitted through the directional dispersion film and incident on the holographic elements, and they can be made so that bias does not occur in the wavelength distribution of the diffraction beams in the forward direction (normal direction of the holographic element). As a result, a holographic screen with excellent color reproducibility can be attained.
The principle of this color reproducibility improvement is as described in Unexamined Patent Publication (Kokai) No. 11-202417.
However, in the above prior art hologram screen, although the color reproducibility of the display elements is extremely favorable, there is the problem that brightness is overly reduced. The reason for this is that the directional dispersion film disperses the image beams more than is necessary. Namely, as shown in FIG. 21, the incident beam passes through the direction dispersion film while being continuously dispersed across a wide range. Consequently, the intensity of the beams incident on the holographic elements at substantially the same angle as the previously set angle of incidence of the image beam on the holographic elements is greatly weakened.
As a result, the effectiveness of the holographic elements is greatly reduced.
FIG. 21 shows the intensity distribution at the outgoing angle of dispersed and transmitted beam, where white beam is incident on the diffuser 98 (directional dispersion film) at an angle of incidence of 35°. The intensity ratio of the ordinate indicates the intensity ratio of incident beam 81 to outgoing beam 82 through the diffuser 98 shown in FIG. 22. The outgoing angle of the abscissa indicates the angle of linearly transmitted outgoing beam 821, the greater the angle with respect to the normal line of the diffuser 98 over the outgoing beam 821 being represented as an increase (+).
For the above reasons, when the visibility control film “Lumisty” serving as a directional dispersion film is laminated onto holographic elements, the image brightness is half or less of that prior to lamination, giving rise to the problem that it can only be used in a relatively dark room.